I usually watch the podcasts and stuff like that so hopefully, depsite just being a fan like all of you, I can clarify a few things that supposedly are true, though since i'm not involved I can't be any more sure than you guys lol
Supposedly the dui's were irrelevant in both cases. For starters they needed Michael to kill someone in order to show the madness that he was feeling. They needed to invest the fans in a character so that we would care when the character was killed, even if wasn't a very likable one. Additionally they had reasons for killing off Libby along with her.
Daniel Dae Kim got a dui and isn't dead though he always was and will be more relevant than the other two were. People used to say that Eko was off for also getting a dui yet his traffic violation was overturned and he supposedly asked to be released because his parents both died (according to him that was why he was released but who knows?)
They did spend several episodes letting us start to know them but overall they were in the show as much as Boone was when he was alive. The difference though is that Boone's crew remained mostly alive, at least the main cast members, while the tail end crew didn't.